Many communication systems include HARQ retransmission functionality (often combined with a demodulation and decoding scheme) which is well known in the art.
For example, HARQ functionality may be applied in a system where link adaptation aims to settle the BLER (block error rate) of a transmission at a certain operating point. If, for example, the BLER operating point is 10% BLER (as is often the case in cellular systems) the 10% of the packets received at the initial transmission of a packet fail to be decoded and 90% of the packets are successfully decoded. This indicates that many of the packets that failed decoding are probably still relatively close to be decodable, and the information that was received in the initial transmission of a failed packet may still be worth using. This is utilized by the HARQ system, in which this information is stored (as soft bit values) and a retransmission of the packet is requested. A retransmission of a packet may, for example, comprise the same information as in the previous transmission or it may comprise different or additional information. When the retransmitted packet arrives, it is combined with the stored information and a new decoding attempt is made. In an exemplary HARQ system, this process is iterated until the packet is successfully decoded or until a maximum number of retransmissions have occurred. A HARQ system may provide robustness against inaccuracies and/or delays in the link adaptation algorithm. A HARQ system may provide a basis for efficient low-latency communication, one reason being that only the physical layer (which generally has faster response times than higher layers) is involved in the HARQ protocol.
HARQ schemes are designed to improve receiver performance and to optimize utilization of channel capacity. However, a drawback with HARQ-systems is that a receiver supporting HARQ has to be able to store soft bit values while waiting for a retransmission.
In many HARQ systems, there is a requirement that a receiver should be able to store packets of a particular size (e.g. relating to a transport format in UMTS—Universal Mobile Telecommunication Standard—or in UMTS LTE—Long Term Evolution). In some HARQ systems there may be a requirement regarding the resolution for each stored soft bit value. Furthermore, some HARQ systems may have requirements that a receiver should be able to support a particular number of simultaneous HARQ process. Such and other requirements may lead to that the memory space that need to be reserved for HARQ-processing (e.g. a HARQ buffer) may be quite extensive and may, for example, severely affect the size of hardware implementations. This may be particularly problematic at the terminal side of a communication system, since a terminal often has stringent area and power consumption requirements. Thus, physical implementation of HARQ buffers may be problematic.
This may be particularly pronounced in systems with high throughput and relatively long latency (e.g. UMTS LTE), where the HARQ buffer size may need to be very large compared to the size occupied by other functionalities of a baseband system. UMTS LTE generally support large information block sizes, leading to even larger packets to transmit since the information block is generally encoded by an error correcting code introducing redundancy symbols (often a multiple of the information block size) before transmission, which additionally increases the required HARQ buffer size.
In UMTS LTE as well as in UMTS (HSPA—High Speed Packet Access), methods have been introduced to limit the HARQ buffer size. In UMTS LTE, the concept is referred to as Limited-buffer Rate Matching, and in HSPA it is denoted Two-stage Rate Matching. These concepts were introduced at the expense of an increased effective code rate. However, even with these standardized methods, the size of the HARQ buffers is still a big contributor to the overall chip size (in particular for UMTS LTE, independently of the UE category).
Therefore, there is a need for methods and arrangements that lowers the required HARQ buffer size.